As a man who has dealt with this many several times, let me impart upon you some advice.
Your instincts are right on. He wants to fuck her, and if you let up even the slightest
bit, he will seize any opportunity to get between you and your woman.
His autistic quoting of Nietzsche is him saying to you that he intends to fuck your bitch, but you already knew that I'm assuming.
That being said, giving her an ultimatum will only make you look weak. Jealousy is the product of insecurity, and women loathe
insecure men. He will not quit until he believes he has no chance, so you need to show him that she is yours
When he's around, be extra physical with her. Make it blatant. Be dominant. It'll piss him off something fierce, and he'll get more aggressive in his attempts to take her. Should you have established your worth with your girl already, she'll reject the orbiter and send him packing or relegate him squarely to the friend zone, from which he will recede in due time as he loses any sense of hope.
Alternatively, show her the fucking cringe quote and she'll see the dude is trying to fuck her in typical deceitful "nice guy" fashion. If she doesn't get it then, dump her. She's an idiot and not worthy of your further consideration. Then do everything I suggested from the get go with the next girl and for fucks sake stop being a bitch.
tl;dr Your jealousy is because you're actually insecure as fuck and that's a far bigger problem for your relationship than some orbiter.